Approximately 5.3 million Americas currently suffer from Alzheimer's disease and, in the absence of effective treatments, this number is expected to balloon to over 14 million by the year 2050. Current immunization studies targeting A? pathology have been largely unsuccessful at stabilizing or restoring cognitive deficits, suggesting that they do not target the proper species: tau. By the time patients are showing signs of dementia, tau pathology is likely to be the principal driver of disease, not A?. A major limitation to current tau antibody therapeutic strategies is the reliance on linear epitope binding for screening, selection, and production of novel antibodies. In this manner, physiologically-relevant tau species may not be targeted. My central hypothesis is that the field of anti-tau antibody development can be improved by two separate means: by employing novel screening technology for selection of monoclonal antibodies and by optimizing immunization antigens for production of monoclonal antibodies. The main goals of this proposal are 1) to determine whether a correlation exists between efficacy of blocking tau seeding in vitro and functional recovery in vivo, as suggested in a recent study by the Diamond and Holtzman Laboratories (Yanamandra et al., in press) and 2) to investigate novel tau aggregates for animal immunization. Aim 1 will test if inhibition of tau seeding in vitro predicts in vivo efficacy. A lage panel of pre-existing antibodies will be screened for their capacity to inhibit tau seeding using two separate aggregation detection assays. Six antibodies will be chosen, over a range of efficacies, for in vivo administration into mid- to late-age P301S mice. After either 6 or 18 weeks of treatment, animals will be assessed pathologically (with immunohistochemistry and ELISA), behaviorally (with fear conditioning), and synaptically (with functional connectivity and optical intrinsic signal imaging) to determine if tau seeding inhibition is predictive of functional recovey. Aim 2 will develop novel anti-tau antibodies from a panel of tau antigens. Recombinant or human-derived tau aggregates of variable sizes (large fibrils, trimer, ~10mer, ~20mer, ~40mer) will be used to immunize mice, and resultant antibodies will be screened for antigen-binding capacity and tau seeding inhibition. Per antigen, six antibodies will be selected for purification and use in future studies outside the scope of this proposal. Cumulatively, it is expected that thi proposal will produce antibodies superior to those currently available because of our innovative approach for targeting tau seeds or seeding activity, rather than linear epitopes. The completion of these Aims will have a significant and direct impact on the development of future novel AD therapeutics.